Licensed Assisted Access (LAA), based on Carrier Aggregation (CA), has been proposed in Long Term Evolution (LTE) systems to utilize additional bandwidth in unlicensed spectrums. FIG. 1 is a schematic diagram showing LAA-LTE. As shown, a terminal device 102, also known as User Equipment (UE) in LTE, is served by a Primary Cell (PCell) 104 operating in a licensed frequency band and a Secondary Cell (SCell) 106 operating in an unlicensed frequency band. The PCell 104 exchanges with the terminal device 102 essential control messages and also provides always-available robust spectrums for real-time and/or high-priority traffic. The PCell 104 also provides mobility management for the terminal device 102 via an LTE radio access network. The SCell 106 can be utilized as a bandwidth booster to serve, e.g., best effort traffic. The SCell 106 typically operates in a downlink-only mode.
In order to co-exist with Wireless Local Area Network (WLAN) and other radio communication systems operating in unlicensed spectrums, an Access Point (AP) associated with the SCell 106 incorporates a particular channel management mechanism. FIG. 2 shows a simplified signaling sequence for this mechanism in the AP. The AP includes a Digital Unit (DU) 202, also known as a Base-Band Unit (BBU), and a Radio Unit (RU) 204. As shown, at 2.1, the DU 202 selects a candidate channel by performing e.g., channel scanning. At 2.2, the DU 202 indicates the selected candidate channel to the RU 204. Then, at 2.3, the RU 204 applies Clear Channel Assessment (CCA) by performing Listen Before Talk (LBT) on the candidate channel to determine whether it is occupied or clear for transmission.
If the CCA succeeds, the RU 204 can send a reservation signal generated by the DU 202 over the candidate channel to occupy the channel. On the other hand, if the CCA fails (e.g., when no CCA succeeds before a predefined timer expires or the number of CCA failures has reached a predetermined value) at 2.4, the RU 204 sends a CCA failure report to the DU 202 at 2.5 to inform the DU 202 of the failure. Then, at 2.6, the DU 202 reselects a new candidate channel and indicates it to the RU 204 at 2.7. At 2.8, the RU 204 applies CCA on the new candidate channel to determine whether it is occupied or clear for transmission. If the CCA succeeds this time, the RU 204 can send a reservation signal generated by the DU 202 over the new candidate channel. If the CCA fails again, the RU 204 will send another CCA failure report to the DU 202 to request another candidate channel.
It can be seen from above that, when a CCA failure occurs on a candidate channel, the RU 204 needs to report the failure to the DU 202 and wait for a new candidate channel indicated from the DU 202. This causes a significant delay in the CCA procedure, particularly when there is a large distance between the DU 202 and the RU 204. Such delay may greatly degrade the performance of the LAA-LTE system as it reduces the chance that the AP can seize an unlicensed channel in competing with other co-existing systems.
There is thus a need for an improved solution for CCA.